Process for absorption of SO2 into phosphate absorbent

ABSTRACT

Higher loadings of SO 2  into an aqueous phosphate absorbent are achieved by use of a pH of no less than about 4.5 and a molarity (e.g., 3.0 or above) which produces a greater SO 2  absorption than expected.

BACKGROUND OF THE INVENTION

In U.S. Pat. No. 3,911,093 to F. G. Sherif et al., a process for reacting SO₂ and H₂ S in an aqueous phosphate absorbent is described in which elemental sulfur is produced. The molarity for the phosphate buffer is said to range from 0.5-2.0 with an exemplification of a maximum of 1.5 molar. The pH range is given as 2.5 to 5, preferably 2.8 to 4.5.

SUMMARY OF THE INVENTION

Unexpectedly, it has been found that at pH values at no less than about 4.5, it is possible to produce a higher SO₂ loading than expected if the molarity and pH values of the phosphate absorbent are suitably adjusted to achieve such a result. In general, use of higher molarities allow the enhanced effect to be seen at pH values closer to 4.5, whereas use of lower molarities require the use of higher pH values. The SO₂ loadings observed are greater than the calculated values for such conditions.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The phosphate absorbent process to which the current invention is an improvement utilizes an alkali metal phosphate absorbent for SO₂ gas. The phosphate absorbent preferably comprises a mixture of Na₂ HPO₄ and NaH₂ PO₄ as shown in the Sherif et al. patent. The absorption temperature can range from about 10° C. to about 95° C.

In accordance with the present invention, the pH of the absorbent is kept at no less than about 4.5, preferably from about 4.5 to about 7.0 and the molarity of the phosphate absorbent is selected so an unexpected increase in SO₂ absorption is achieved. At lower molarity values, e.g., about 1.0, the pH needs to be somewhat higher (e.g., about 5.5) than if higher molarity values are used (e.g., 3.0 or higher) in which an enhanced effect is also seen at somewhat lower pH values as well (e.g., down to about 4.5). In general, the greatest enhancement in SO₂ absorption is seen as the molarity of the absorbent is increased and as the pH is increased. The greatest percentage increase of SO₂ absorbent is noted for more dilute SO₂ -containing streams.

The SO₂ -rich absorbent resulting from the absorption of SO₂ into the absorbent is then treated with H₂ S to form sulfur, the sulfur is separated from the solution, and the lean absorbent is recycled to the SO₂ absorption step as described in U.S. Pat. No. 3,911,093.

The following Examples illustrate the present invention in more detail.

EXAMPLE 1

This Example describes the general procedure used to generate the data in the later Examples.

The apparatus used is shown in the FIGURE which forms a part of the present invention. Air and SO₂ were metered through flowmeters and fed through glass lines joined with minimum length tubing (TYGON brand). Stopcocks were used to control the gas flow path. The gases were passed through a vent tube at which point a side stream was taken for measurement of SO₂ concentration by the Reich test which consists of aspirating the gas through a known quantity of standard iodine solution to a starch endpoint.

During an absorption cycle, the gases passed through two absorption flasks which were thermostated in a water bath at the desired test temperature. The first flask contained phosphoric acid, and the second contained phosphoric acid neutralized to the desired initial pH. The solutions in both flasks had the same phosphate molarity. The purpose of the first flask (unneutralized phosphoric acid) was to hydrate the gas mixture so that water (as vapor) would not be removed from the flask containing the buffer solution. A heated scrubber (20.32 cm. hgt. by 2.54 cm. diam.) filled with gas beads removed any phosphoric acid that might have been entrained in the gas stream. Both flasks were equipped with open tube spargers.

A combination pH electrode was inserted in the buffer solution to monitor pH change, since as SO₂ was absorbed, the pH gradually would fall. When the pH was stable for 30 minutes, saturation of the buffer by SO₂ was assumed to be complete.

To avoid loss of SO₂ during sampling for analysis, the flask containing the buffer saturated with SO₂ was rapidly stoppered and cooled to room temperature. A pipet was used to remove samples for testing.

The analysis of SO₂ in the gas stream was done by the Reich test. A slip stream of the gas stream under test was bubbled through an iodine--starch solution (i.e., 1-10 ml. of a 0.1N iodine solution in about 400-600 ml. of water in a 1 liter Ehrlenmeyer flask equipped with a magnetic stirrer). At the disappearance of the iodine-starch color, the volume of gas passed through the solution was measured by means of a liquid filled burette and leveling bulb (previously set to zero at the start of the run). The following equation was used to calculate the SO₂ concentration: ##EQU1## where

N_(I) =the normality of the iodine solution

ml_(I) =milliliters of iodine solution

air=air in burette which represents the volume of gas passed through the iodine solution for decolorization at a nominal 22.5° C.

The appropriate volume of iodine solution is:

    ______________________________________                                         ml of 0.1 N I.sub.2                                                                         Vol. of gas (ml.)                                                                           SO.sub.2 (ppm)                                       ______________________________________                                         1.0          400            3000                                               1.0          150            8000                                               5.0          120          50,000                                               10.0         170          70,000                                               10.0         135          85,000                                               ______________________________________                                    

EXAMPLE 2

This Example illustrates how the calculated values for SO₂ solubility reported in later Examples was measured.

The equation relating concentration in solution of SO₂ and buffer, pH, and dissociation constants for sulfurous acid and buffer acid is given as Equation 1. The dissociation constants for water, sulfurous acid and phosphoric acid are given as Equations 2a to 2c. The relationship between SO₂ concentration in solution and in gas is given as equation 3 along with Henry's constant (equation 3a). ##EQU2## where

K₁ K₂ =dissociation constants for H₂ SO₃

K₃ K₄ K₅ =dissociation constants for H₃ PO₄

Kw=dissociation constant for water

[ ]=molar concentrations

Equation 2a (Dissociation Constant H₂ O) ##EQU3##

For: 0° to 50° C.

Reference: W. Stumm, J. J. Morgan, Aquatic Chemistry, P. 76

Equation 2b (Dissociation Constants--Sulfurous Acid) ##EQU4##

For: 0° to 100° C.

Reference: S. A. Newman, Thermodynamics of Aqueous Systems with Industrial Applications, 1979, pp 139-172 (ACS Symposium Series No. 133)

Equation 2c (Dissociation Constants--Phosphoric Acid) ##EQU5##

    log K.sub.5 =4.8×10.sup.-13

For: 0° to 60° C.

Reference: Mellor, Inorganic and Theoretical Chemistry, Vol. VIII, Supplement III, p. 697.

Equation 3 ##EQU6## Equation 3a ##EQU7##

For: 0° to 100° C.

Reference: Same as Equation 2b

EXAMPLE 3

The solubility of SO₂ in phosphate buffer at 1, 2 and 3 molar at 65.6° C. was measured for various feed gases:

    ______________________________________                                                       Run No.                                                                          1      2        3    4                                         ______________________________________                                         Phosphate Molarity                                                                             1.0    1.0      1.0  3.0                                       Added S.sub.2 O.sub.3.sup.═ (gm.)                                                          0      0        15   0                                         Added SO.sub.4.sup.═ (gm.)                                                                 0      65       65   0                                         % SO.sub.2 in Air                                                                              6.91   8.64     7.88 0.87                                      SO.sub.2 solubility (gm/l.)                                                    Found           10.4   12.2     12.0 6.35                                      Calculated      13.3   15.0     14.3 7.44                                      % Found/Calc.   78.2   81.3     83.9 85.4                                      End pH          2.96   2.90     2.89 3.80                                      ______________________________________                                                       Run No.                                                                          5      6        7    8                                         ______________________________________                                         Phosphate Molarity                                                                             3.0    2.0      2.0  2.0                                       Added S.sub.2 O.sub.3.sup.═  (gm.)                                                         15     0        15   0                                         Added SO.sub.4 .sup.═  (gm.)                                                               65     0        65   0                                         % SO.sub.2 in Air                                                                              8.10   5.06     5.02 4.98                                      SO.sub.2 solubility (gm./l.)                                                   Found           23.9   14.8     15.5 14.8                                      Calculated      24.2   15.6     15.6 15.6                                      % Found/Calc.   98.8   94.9     99.4 94.9                                      End pH          3.22   3.19     3.15 3.16                                      ______________________________________                                    

The addition of S₂ O₃ ═ and SO₄ ═ had no substantial effect on the solubility of SO₂. The correlation between observed and calculated SO₂ values was good. In all cases, the initial pH was about 4.5.

EXAMPLE 4

The effect of phosphate molarity on SO₂ solubility at 65.6° C. was calculated and the following results were obtained:

    ______________________________________                                                          Calc.                                                         % SO.sub.2                                                                           Buffer     SO.sub.2 Solubil.                                                                         % Increase SO.sub.2                                in Air                                                                               Molarity   (gm/l.)    Solubil. over 1 Molar                              ______________________________________                                         1.0   1.0        4.8        --                                                       2.0        6.6        38                                                       3.0        8.0        67                                                 5.0   1.0        11.2       --                                                       2.0        15.5       38                                                       3.0        18.8       68                                                 8.0   1.0        14.3       --                                                       2.0        19.9       39                                                       3.0        24.1       68                                                 ______________________________________                                    

EXAMPLE 5

SO₂ solubility was measured at 65.6° C. at an initial pH of 4.6-4.73 as follows:

    ______________________________________                                                      Run No.                                                                        1        2       3                                                ______________________________________                                         Phosphate Molarity                                                                            5.0        5.0     7.5                                          % SO.sub.2 in Air                                                                             0.84       7.24    7.88                                         SO.sub.2 Solubil. (gm/l.)                                                      Found          17.2       35.1    47-50*                                       Calculated     9.2        29.0    37.7                                         % Found/Calculated                                                                            187        121     130                                          Final pH                                                                       Found          4.13       3.58    3.98                                         Calculated     3.83       3.38    3.46                                         ______________________________________                                          *presence of crystals of sodium phosphate precluded more accurate              analysis.                                                                

A positive deviation from calculated values was observed at initial pH values about 4.5, i.e., for 5.0 and 7.5 molar. The greatest degree of positive deviation was observed for the most dilute SO₂ gas stream (Run No. 1).

EXAMPLE 6

The SO₂ absorption from a 3000 ppm gas stream at an initial pH of about 4.5 and 23.9° C. was measured at molarities of about 1-5 as follows:

    ______________________________________                                         Run No.     1      2       3    4     5    6                                   ______________________________________                                         Phosphate Molarity                                                                         1      2       3    3.5   4    5                                   SO.sub.2 Solubility                                                            (gm/l.)                                                                        Found       2.5    5.2     10.8 10.9  18.3 17.6                                Calculated  3.5    4.9     5.9  6.4   6.8  7.5                                 Final pH                                                                       Found       3.23   3.56    3.99 3.90  4.06 4.21                                Calculated  3.49   3.64    3.72 3.75  3.78 3.82                                ______________________________________                                    

These data illustrate the positive deviation from calculated values for SO₂ absorption when buffers above two molar were used and when a relatively dilute gas stream is used (i.e., one more dilute than any used in Example 3).

EXAMPLE 7

Solubility data were developed for three molar phosphate absorbents at 65.6° C. for an air feed containing about 7.78% SO₂ :

    ______________________________________                                         Initial pH        SO.sub.2 Absorbed (gm/liter)                                 Run   Ambient  65.6° C.                                                                           Found   Calculated                                   ______________________________________                                         A     6.00     5.84       88.0    29.5                                         B     5.50     5.56       64.8    26.2                                         C     5.00     5.10       44.0    24.7                                         D     4.50     4.69       23.9    24.2                                         ______________________________________                                    

These data illustrate the positive deviation from calculated values for SO₂ absorption at pH values above 4.5 for 3 molar phosphate buffer solutions. A greater degree of SO₂ absorption was observed for the higher pH values.

EXAMPLE 8

This Example illustrates the results obtained when a 3000 ppm SO₂ gas stream was treated with 3 molar phosphate absorbent at 48.8° C. and an initial pH of 6.0. The calculated and actual SO₂ loadings were:

    ______________________________________                                                     SO.sub.2 (gm/l.)                                                   ______________________________________                                         Calculated    14.46                                                            Found         61.5                                                             ______________________________________                                    

This illustrates use of the present invention with a relatively dilute SO₂ gas stream at an initial pH of 6.0 with three molar phosphate.

EXAMPLE 9

This Example illustrates the solubility of SO₂ in phosphate buffer at about 65.6° C. under a variety of conditions. The results are as follows:

    ______________________________________                                                                      SO.sub.2 Absorption                               Phosphate                                                                               SO.sub.2 Conc.                                                                           Initial   (gm/l.)                                           Molarity ppm       pH        Found Calculated                                  ______________________________________                                         1.0      75,000    5.0       12.7  14.2                                                           5.5       18.4  14.7                                                           6.0       26.6  16.2                                        2.0      74,000    5.0       25.3  19.7                                                           5.5       35.8  20.7                                                           6.0       51.2  23.7                                        3.0      78,000    5.0       44.0  24.7                                                           5.5       64.8  26.2                                                           6.0       88.0  29.2                                        ______________________________________                                    

The above data illustrate the positive deviation from calculated SO₂ absorption obtained for a number of the runs. The use of lower molarity phosphate solutions (e.g., 1.0 molar) dictates use of a somewhat higher pH (e.g., 5.5) than needed if higher phosphate molarity is employed.

The foregoing Examples illustrate certain embodiments of the claimed invention and should not therefore be construed in a limiting sense. The scope of protection that is sought is set forth in the claims which follow. 

What is claimed is:
 1. A method of absorbing sulfur dioxide wherein sulfur dioxide is absorbed into an absorbent comprising an aqueous solution of an alkali metal phosphate, wherein said method comprises utilizing a pH for the phosphate of no less than about 4.5 and adjusting the molarity of the phosphate absorbent so as to yield an increased absorption of sulfur dioxide in the absorbent.
 2. A method as claimed in claim 1 wherein the pH ranges from about 4.5 to about 7.0.
 3. A method as claimed in claim 1 wherein the molarity of phosphate is about 1.0 or above.
 4. A method as claimed in claim 1 wherein the molarity of phosphate is about 3.0 or above.
 5. A method as claimed in claim 1 wherein the pH ranges from about 4.5 or above and the molarity of phosphate is about 3.0 or above.
 6. A method as claimed in claim 1 wherein the pH ranges from about 5.5 or above and the molarity of phosphate is about 1.0 or above. 